1. Field of the Invention
The present invention relates to an exposure control device for camera, which is capable of controlling exposure amount by changing over aperture size in three steps according to subject luminance.
2. Background Arts
A lens-fitted photo film unit (hereinafter referred to as film unit) is well-known as a simple camera which is loaded with an unexposed photo filmstrip, and in which a simple exposure mechanism is incorporated. In order to lower the manufacturing cost, the film unit is provided with a shutter blade of knocked type, so the shutter speed is fixed at a predetermined value. Also, the film unit is provided with a taking lens of fixed-focus type. To ensure pan-focus by use of such a taking lens, aperture size is designed to be relatively small, compared to other camera like a compact camera. Moreover, for the purpose of miniaturization of the film unit, a main capacitor of an internal flash unit has relatively small capacitance, so light amount of flash light is smaller that that of other cameras.
The film unit has such a characteristic that everyone can photograph easily at any time and situation, so the film unit is targeted amateur users who are not familiar with operation of a camera. In addition, the film unit is presumed to be used in a condition ranged from a low luminance level such as indoor photography (light value LV is about 5.5), to a high luminance level such as fine outdoor photography (light value LV is about 15). In order to obtain adequate exposure amount in various conditions of wide luminance range, it is considered to use a photo filmstrip with high film speed such as ISO 1600 type or ISO 3200 type.
Moreover, for the purpose of photography with proper exposure amount under various conditions of wide luminance range, it is considered to provide a film unit with an exposure control device for adjusting exposure amount in accordance with subject luminance by changing over aperture sizes in two steps. There are two types of exposure control devices, one is an automatic type for changing over aperture size automatically, and the other is an interlocked type in which aperture size is changed over in accordance with operation of flash switch.
The exposure control device of automatic type has a photometry element such as a cadmium sulfide (CdS) or a photo diode. When the subject luminance is equal to or more than a predetermined level, the exposure control device moves a stop changeover plate automatically to insert a small stop opening, smaller than a normal one, on an photographic light path PL. The exposure control device of automatic type is used for decreasing exposure amount by setting the small stop opening on the photographic light path PL, to prevent over exposure in the high luminance level.
The exposure control device of interlocked type has a changeover plate that moves subsidiary to the movement of the flash switch. When the flash switch is moved to an ON position, the stop changeover plate is moved to insert a large stop opening, larger than the normal one, on the photographic light path PL. The exposure control device of interlocked type is used for preventing under exposure in photography in the low luminance level. As mentioned above, the film unit has a main capacitor of small capacitance and a small-size stop opening, so under exposure is likely to occur in photographing a subject of the low luminance level. It is assumed to turn on the flash device in a low luminance condition, so the exposure control device of interlocked type is designed to set the large stop opening to increase exposure amount when the flash switch is moved to the ON position.
In the exposure control device of automatic type, however, when the size of the stop opening, chosen in a low luminance condition, is enlarged for the purpose of preventing under exposure, there is a problem to make the depth of field smaller, which causes lack of focus. On the other hand, the exposure control device of interlocked type, it is not possible to conduct daylight synchronized flash photography that is effective in photography against the sun, since extremely over exposure is happened by flash photography in daylight condition. Turning on a flash unit in the high luminance level surely causes over exposure. This problem becomes worse especially in using a photo filmstrip of high film speed.
In a middle luminance level (light value LV is from 8 to 13), such as an outdoor photography in an evening, an indoor photography under relatively bright light, and a daylight photography in cloudy or rainy condition, there are a first luminance range in which flash light is needed, and a second luminance range in which flash light is unnecessary. The border of these two ranges is changed subtly in accordance with aperture setting or the like. Thus, it is difficult to judge whether or not to use a flash unit. In addition, the flash switch of the film unit is usually operated manually, so judgment to use the flash unit is left to a user.
In the middle luminance level, failure judgment or operation is likely to cause over or under exposure even when the exposure control devices of both type are applied. It is difficult in the middle luminance level to judge whether or not the flash unit is required, so the user may judge that the flash photography is not necessary, although flash light is practically required. That is, there is a gap between a luminance level where the user needs flash light, and a luminance level to require flash light theoretically.
In order to prevent over exposure in the high luminance level, the exposure control device of automatic type is designed to set the small stop opening on the photographic light path PL, so the stop opening of normal size is usually set in the middle luminance level. In that case, flash photography may cause over exposure dependent upon subject luminance, in the middle luminance level. On the other hand, the exposure control device of interlocked type is designed such that a large stop opening is not set on the photographic light path PL unless the flash switch is not located at ON position. Thus, in the middle luminance level where flash light is needed, if the user forgets moving the flash switch or judges that flash light is unnecessary, under exposure may be happened.
Frequency of photographing a subject of middle luminance level is not quite a few, so it is desirable to carry out proper exposure control easily and surely in the middle luminance level. In addition, in consideration of the characteristic of the film unit that everyone can take a photo readily, to force an user a difficult judgment is not preferable, since it causes worse management of the film unit.
In view of the foregoing, an object of the present invention is to provide an exposure control device for camera that is capable of proper photography in wide luminance range from a low luminance level to a high luminance level.
To achieve the above objects, an exposure control device for a camera, having an open aperture set on a photographic light path, is comprised of a first stop member with a small stop opening that is smaller than the open aperture, a second stop member with a middle stop opening that is larger than the small stop opening and smaller than the open aperture, photometry means for measuring subject luminance, first stop changeover means for moving the first stop member, and second stop changeover means for moving the second stop member. The first stop member is movable between a small aperture position where the small stop opening is set on the photographic light path, and a first large aperture position where the small stop opening is retracted from the photographic light path. The second stop member is movable between a middle aperture position where the middle stop opening is set on the photographic light path, and a second large aperture position where the middle stop opening is retracted from the photographic light path. The first stop changeover means sets the first stop member either said small aperture position or the first large aperture position in accordance with subject luminance measured by the photometry means. The second stop changeover means sets the second stop member either the middle aperture position of the second large aperture position in accordance with selecting operation to determine whether or not to carry out flash photography.
In the preferred embodiment, the camera has a taking lens, which includes a plurality of lens elements. One of the first stop member and the second stop member is disposed between the lens elements, and the other stop member is disposed behind the taking lens. The taking lens is movable between a short-distance position where the subject distance is ranged from 1.5 m to 2 m, and a long-distance position where the subject distance is ranged from 4.5 m to 7 m. The taking lens is set at said short-distance position in choosing flash photography, and set at long-distance position when flash photography is not selected.
The second stop member is retained at the middle aperture position, and the second stop changeover means moves the second stop member to the second large aperture position in accordance with operation to drive a flash unit. The first stop member is kept at the small aperture position, and the first stop changeover means moves the fist stop member to the first large aperture position when subject luminance is less than a predetermined value. The photometry means is driven by shutter release operation.
The camera is a lens-fitted photo film unit having a photo filmstrip and a shutter device of same shutter speed. The film speed of the photo filmstrip is more than ISO 1600. The f-number of the open aperture is set within a range of 4.0 to 5.6, the f-number of the middle stop opening is set within a range of 8 to 11, and the f-number of the small stop opening is set within a range of16 to 27. The predetermined level of subject luminance is determined within a range of 10.5 to 11.5 in light value. The lens-fitted photo film unit has recording means for recording open-aperture information on a marginal area of a corresponding frame of said photo filmstrip.